Three-dimensional laser velocimeter (3D LV) instruments are used to determine the three orthogonal components of velocity (u, v and w) in a fluid flow field. These instruments are composed of three independent one-dimensional laser velocimeter (1D LV) instruments oriented so as to measure different components of fluid velocity (u.sub.1, u.sub.2 and u.sub.3) which are not necessarily orthogonal. The measurements of the three independent 1D LV channels are then combined by way of a coincidence logic means to form the 3D LV measurement system.
Each of the 1D LV instruments is associated with an ellipsoidal probe volume created in space by laser and optical means. Within this probe volume, a set of planar fringes (two or more) are formed optically with known spacing (d.sub.1, d.sub.2 and d.sub.3 fringe spacing for the three independent channels, respectively) and orientation normal to the velocity component measured by the associated 1D LV channel. If a particle travels through a 1D laser velocimeter probe volume, light will be scattered by this particle at an intensity that will vary in time with a frequency that will depend on the fringe spacing and the particle velocity (i.e., fringe crossing time of t.sub.d,1 =d.sub.1 /u.sub.1 for channel 1). This light can be detected by means of a photodetector device and its frequency content measured. These scattering particles arrive at random times and are not always present in the probe volume. The photodetector output signal will thus consist of intermittent "bursts" during which time a scattering particle is present in the probe volume and for which time the photodetector output signal may be analyzed for frequency content and the 1D component of particle velocity thereby deduced.
Upon successful measurement of the 1D component of a single particle's velocity by a 1D LV, an event pulse and a digital word or other form of encoded data related to the particle velocity will typically be presented. It is assumed with some justification that the particle travels with the fluid, and the particle velocity is a reasonable approximation of the fluid velocity.
To form the 3D laser velocimeter system, three probe volumes are created optically in space. Each of these probe volumes is associated with an independent 1D laser velocimeter measurement channel. In FIG. 1, we show the typical case where two of the 1D LV systems have their associated probe volumes overlap entirely or nearly entirely (probe volume labeled 20), but with their planar fringes oriented so the velocity components measured by these two 1D LV systems are independent (and often orthogonal). In order that the third measured velocity component be independent of the other two, the ellipsoidal probe volume (labeled 18) associated with the third 1D LV system must be canted at an angle to the probe volumes associated with the other two channels. Hence the third probe volume will overlap only partially with the other two probe volumes if three independent components of velocity ar to be measured.
As a result of this partial overlap and also of possible unsuccessful measurements in any of the laser velocimeter channels, it is necessary to accomplish a coincidence or simultaneity check on the event pulses originating from each of the three 1D laser velocimeter channels in an attempt to ensure that the three measurement systems are reporting data actually obtained from the same particle.
The conventional prior art technique for checking coincidence of the event pulses is the use of a fixed coincidence time window. In this concept, one of several (e.g., three) 1D LV measurement devices will generate a channel event pulse at a time t.sub.0 indicating that particular channel has accomplished a measurement. A time window of fixed but operator adjustable duration t.sub.c starts at time t.sub.0 during which the remaining channel measurement devices must also generate a channel event pulse. If each of the remaining channels generate a channel event pulse during the time interval (t.sub.0, t.sub.0 +t.sub.c), then a system event pulse is generated. If one or more of these channels do not generate a channel event pulse during this time interval, then no system event pulse is generated and the data from all channels is disregarded. If the system event is generated, the system event is used to initiate the acquisition of the data from each of the several (e.g., three) 1D LV measurement devices. An additional feature of this prior art is that, once a channel has generated a channel event, further measurements and channel events from that particular channel are inhibited until either the coincidence window time has elapsed or until the system event is generated and the data acquisition system has acquired the desired data from the various measurement devices. In this prior art approach, the individual measurement channels essentially function independently of each other and do not interact except for the above coincidence time window test and the above inhibit function. Further details on the fixed coincidence time window concept are provided in Dean Harrison et al., pending NASA U.S. patent application Ser. No. 725,714, filed Apr. 25, 1985, now U.S. Pat. No. 4,779,222.
Frequency measuring devices exist which are specially designed for measuring the frequency or time period content of the intermittent bursts present in the analog signal of a 1D LV photodetector output. These specialized devices include a spectrum analysis device, a tracking frequency-to-voltage converter and an electronic counter.
In the counter form of these specialized devices, the analog photodetector signal is converted into a digital pulse train, the pulses being related to the passage of the particle through the fringe pattern formed in the probe volume. An additional digital signal may be generated which is "true" when a particle is present in the probe volume and "false" otherwise. This signal may be referred to as the channel measurement burst signal. The digital pulse train may be analyzed for the time period between pulses while the measurement burst is "true". A 1D LV channel event is then generated and 1D LV data (either in digital or analog form, but typically digital) related to the time period or frequency of the digital pulse train is then presented and made available for data acquisition. Additional 1D LV validation schemes, such as a minimum number of pulses in the 1D LV digital pulse train, may be carried out to determine if the event pulse should be generated and if the data output should be updated with the latest data. The frequency tracker form of these frequency measuring devices is similar to the counter, in that a channel event pulse is generated when valid 1D LV data is available, and the data is updated with the latest validated data. The frequency tracker differs from the counter by using a frequency-to-voltage conversion instead of a digital pulse train.
Another form of 1D LV frequency measuring device uses high-speed analog to digital conversion of the 1D LV photodetector signal, which is then stored in a bank of physical memory. The data in physical memory may be processed for presence of a particle in the 1D LV probe volume (analogous to the measurement burst signal) and for frequency content of the photodetector signal (related to the fringe crossing rate of the particle in the 1D LV probe volume) by a CPU using various numerical algorithms.
Further indication of the state of the art in laser Doppler velocimetry and related measuring devices, including the above described techniques, is provided by the following issued U.S. Pat. Nos.: 3,711,200, issued Jan. 16, 1973 to Maughmer; 3,743,420, issued July 3, 1973 to Iten et al.; 3,860,342, issued Jan. 14, 1975 to Orloff et al.; 4,148,585, issued Apr. 10, 1979 to Bargeron et al; 4,373,807, issued Feb. 15, 1983 to Gouesbet; 4,506,979, issued Mar. 26, 1985 to Rogers and 4,527,894, issued July 9, 1985 to Goede et al, and in William D. Gunter, Jr. et al., pending NASA U.S. patent application Ser. No. 846,427, now U.S. Pat. No. 4,697,922, filed Mar. 31, 1986.
While laser Doppler velocimetry is clearly a well developed art, a significant and unrecognized source of error for the three-demensional implementation has been discovered with prior art simultaneity detection. Further improvement in simultaneity detection is therefore required in order to avoid this error.